If you can get your hands on one, there are "sniffers" out there that gas line technitions use to zero in on a leak. They basically re-set their zero as you go, so as you get closer and closer to the source, the sensor becomes more and more accurate. A bit like playing "warmer" "Colder", except with a gadget.
Their kind of hard to get though, so you might just be better off looking for "damp dust" along the lines, and dont forget the top of the tank where the sensors etc go in.